It is often desirable to automatically and repeatedly dispense a predetermined volume of a material. This can be done manually by using a variety of measuring devices. Such devices include, but are not limited to, graduated cylinders, volumetric flasks, graduated pipettes, and weight scales. However, these methods must be performed manually by an individual and are both inefficient and time consuming. Therefore, several methods have been developed for automatically measuring and dispensing a predetermined amount of a material.
It is common to use a pump that is programmed to run for a predetermined length of time in order to dispense a set volume of material. Alternatively, piston-type pumps may be programmed to perform a single stroke when activated, thereby dispensing a volume of material equal to the volume displaced by the piston.
Dispensing chambers have also been developed that have an integrated weight scale. Material is added to the chamber until a particular, predetermined weight is reached. The material may then be dispensed from the chamber.
Direct volume measurement is also used in volumetric dispensing devices, particularly for measuring fluid materials. An accumulation chamber of a predetermined volume has a sensor at the top of the chamber. Material is added to the accumulation chamber until the level of the material reaches the level of the sensor and is detected. Upon detection, the addition of material to the chamber is halted and the material in the chamber is dispensed.
Volumetric dispensers have become increasingly common. However, a problem often encountered is finding a proper detection sensor for the top of the accumulation chamber. Photoelectric sensors are a common type of detector used. However, especially with granular solid material, they are not always accurate. Small granular material is often susceptible to both static electricity and caking due to ambient moisture. A small amount of the material may adhere to the sides of the accumulation chamber, thereby blocking the light emitted by the sensor. Thick liquids may also temporarily coat the walls of an accumulation chamber and make photoelectric measurement unreliable.
Electric current detection has also been used. When a conductive liquid reaches a sensor at the top of an accumulation chamber, a current may be detected at the level of the sensor. However, many materials are not conductive. Additionally, those that are may be damaged or altered by the application of an electric current.
Float type level switches have also been used. For example, toilets use a float type level switch to regulate the volume of water in its reservoir. However, float sensors are unusable for level detection of fluid solids.
It is therefore desirable to provide an accurate sensor for detecting when a fluid solid or a liquid has reached a certain point in an accumulation chamber.